1. Field of the Invention
The present invention relates to an illumination device that illuminates a display portion of a control knob provided in, for example, an on-vehicle control panel.
2. Description of the Related Art
In control panels used in on-vehicle air conditioning systems and audio systems, an operator (occupant) selectively presses or rotates a plurality of control knobs, and an inherent function assigned to the control knob is thereby performed. Normally, the control knobs have display portions corresponding to their respective assigned functions. An illumination device is mounted so as to allow the display portions to be viewed even in an unlit environment.
Japanese Unexamined Patent Application Publication No. 2004-228022 discloses an illumination device in which light sources (LEDs) are provided in a tubular case disposed on the back side of a control knob, and in which a flat light guide plate is fixed to holding bars integrally provided with an upper end of the tubular case. Light emitted from the light source is diffused by the light guide plate and is then applied to the back side of the control knob, thereby displaying a design, such as characters and images, on a top face of the control knob.
The above-described publication does not teach a specific means for fixing the light guide plate to the holding bars of the tubular case. For example, when an adhesive, which is known as a general fixing means, is used, a step of applying the adhesive is troublesome, and the adhesive strength declines with time. Accordingly, as a means for fixing the light guide plate without using an adhesive, a holding structure can be adopted in which a stepped portion is provided at an upper end of an inner surface of the tubular case, and projections are provided on the back side of the control knob. The control knob is combined with the tubular case by snap fitting so that the outer edge of the light guide plate is clamped by the stepped portion of the tubular case and the projections of the control knob.
In this holding structure, it is necessary to properly set the distance between the stepped portion and the projections in consideration of the thickness of the light guide plate. However, since the control knob having the projections is combined with the tubular case having the stepped portion by snap fitting, if the distance between the stepped portion and the projections is smaller than the thickness of the light guide plate within the dimensional tolerance, it is quite difficult to snap-fit the control knob and the tubular case. For this reason, the distance therebetween needs to be sufficiently larger than the thickness of the light guide plate. As a result, the light guide plate rattles between the stepped portion and the projections, thus causing an abnormal noise during driving.